The field of the invention is corona treatment stations, and particularly, the construction of electrodes for such stations.
Corona treatment stations for sheets, or webs of plastic materials take many forms. Typically, the material to be treated is fed through a treatment zone in which one surface of the material is bombarded with ions produced by a high voltage alternating electric field. The material is supported in the treatment zone by a conductive roller which also serves as one electrode for the corona treater. A second, active electrode is supported in the treatment zone and is spaced from the conductive roller equidistantly along its length. For many years the active electrode has taken the form of metal segments pivotally attached to a bar or beam, so as to be selectively swung away from the roller for removal from the treatment zone. In this manner, the width of the treatment zone can be adjusted to accommodate webs of different width.
One difficulty is that such segmented electrodes produce an uneven treatment. This may be caused either by an uneven spacing of the segments from the roller electrode or by changes in the electric field strength at the junction of adjacent segments.
Treatment station reliability has been significantly increased recently by employing an elongated active electrode having a coating, or jacket, made of a heat resistant insulating material. Such a structure is disclosed in U.S. Pat. No. 4,446,110 entitled Treating the Surface of Articles by Electrical Brush Discharge, where the active electrode is constructed from a hollow quartz rod which is filled with a conductive material and which extends the entire length of the roller electrode. While such constructions provide relatively uniform treatment, the width of the treatment zone is fixed by the length of the active electrode.
A number of attempts have been made to provide a segmented active electrode in which each segment is covered with an insulating jacket. Characteristically, such structures have employed a metal bar or beam which extends along the length of the roller electrode and which supports a series of metal fingers that each extend from the beam toward the roller electrode. In this respect they are very similar to the well known segmented metal electrodes in that the separate metal fingers can be removed or swung away to adjust the width of the treatment zone. The difference is that a jacket of insulating material is slipped over the end of each metal finder to allow bare roll treating. The uneven treatment produced by segmented active electrodes is greatly amplified when the segments are insulated in this manner, and such constructions thus far have not been commercially successful.
U.S. Pat. No. 4,575,379 entitled Electrode Element For Corona Treater describes another approach to providing an insulated active electrode which is segmented to allow alteration of the treatment zone. Individual insulated segments are mounted to a track and selectively connected to a conductive electrode. Uneven treatment is reduced by angling the ends of each insulated segment. While the uniformity of treatment is improved with this construction, it is still uneven and the individually shaped segments are costly to manufacture.